I need to know if someone has experience in designing low temp freezer for -76F (-60C).
Also I am curious to know:
1. What kind of defrost to be used -- Hot Gas or Electric. Will there be a big shock in the system with HG defrosting. This is a 1 evap system and during defrosting, there will be no cooling a/v for intermediate stage.
Can we still use HG defrosting for 2 stage?
2. Do we need to use any special metal for indoor unit enclosure -- Can alumnium/stainless withstand -76F temp?
3. Will 2 stage or cascade will be better for the application?
4. Accumulators and other components are rated for -40F -- can they be used for -76F?
5. How many Fins/Inch are required for Evap?
6. What are the other parameters needs to be considered for this low temp application.
7. What will be the problems with comp running in vacumm?
I need -76F room temp.
-95F SST / 99F Condensing is the opearting limits for the 2 stage compressor I selected Bitzer 30 HP will deliver 20,000 BTU @ -95 SST.
Do U think 2 stage will work -- it would be simpler than cascade?
The first thing you will need to find out is what the maximum evaporating temperature of the compressor. All compressors have a max evap temp, but on low temp compressors it can have a huge impact on design (the whole reason for a crankcase pressure regulator). You will need to keep this in mind for either a simple direct expansion(the bitzer you have chosen), or a cascade.
1)Defrost: Although hot gas is an energy saver, you would run into problems with it being a single system, and that low of a temp. So my suggestion is to stick with elec.
2)Metal: Check the specs. I would be concerned with temperature swing as much as the temperature itself.
3) If you can find a compressor to do it in a single circuit, it will be easier, cheaper, etc. But be careful of specs, refrigerant choice (vacuum), defrost, etc. The cascade would be more efficient, less stress on the compressor, but would need to be balanced, and good control.
4) You will find that almost all components are rated only to -40. It costs money to rate lower, and few people need lower. And the suppliers official response will be don't go below the rating. However, read between the lines. Most of this equipment is made against the recommendations of the suppliers.
5) For the evap, the fins per inch is not as much the issue, but how easily the ice will form/slide off, and of course btu.
6) Biggest concerns are control, balance if it is a cascade/vacuum if it is a single. And defrost! But there is a lot in the details.
7) The compressor running in a vacuum is not the issue. It is leaks pulling in air/water. But on a cascade you can have a second refrigerant that would go to -100 and still not be in a vacuum.
I don't have time right now to respond to all, but want to make sure you understand the operating envelope of the compressor, because it cannot be what you have stated. -95 min suction temp makes sense... +95 max discharge temp also makes sense... however there also has to be a max suction temp, and a min discharge temp.
The reason why this needs to be researched first, is that I would guess that the max suction temp for the compressor is below freezing. That is when a cpr becomes necessary for after defrost.
Call bitzer and ask for the full operating envelope, in fact if you decide to keep going on this you will probably need to call quite a few people.
Sorry but design is not my area of knowledge. Engineers design and I implement the design. Also my experience is solely with large Freeze Dryers. Anyway back to the ETV we do not utilize any with MOP feature and the one's we use are controlled by a PLC so the valves are not specific to refrigerant type as the PLC will control how much the valve should open. I did however notice that all of our mechanical txv's are for 507/404A. The curves are very similar so this makes sense.